The present invention relates to a nonvolatile semiconductor memory or a programmable read only memory in which each of the memory cell is composed of an insulated gate field effect transistor having a floating gate, and more particularly to a writing or programming circuit in such a memory.
A writing circuit in a semiconductor memory of the above-mentioned type includes a writing transistor connected in series to a selected one of memory cells to supply a programming voltage and current thereto. In the prior art, the writing transistor and the memory cell transistor are of the same conductivity or channel type. Therefore, the source of the writing transistor is connected to the memory cell transistor. The memory cell transistor is supplied with the programming voltage and current, so that electrons or holes are injected into the floating gate of the memory cell transistor. A programmed memory cell is thus obtained. The programmed memory cell continues to be supplied with a relatively high voltage responsive to the programming voltage for a programming period. For this reason, a channel breakdown occurs in the memory cell transistor to lower the potential at the connection point of the writing transistor and the memory cell transistor. In other words, the programmed memory cell has a negative resistance characteristic. The source potential of the writing transistor is thereby decreased to increase the current flowing through the writing transistor and the memory cell transistor.
Thus, the semiconductor memory including the prior art writing circuit consumes a considerably large programming power.